The present invention generally relates to a method and an apparatus for the measurement of object height and volume using a combination of optical means and image processing techniques.
There are many applications that require the measurement of height and volume using non-contact means. For example, a first step in a surface-mount printed circuit board assembly process is the printing of solder paste onto the solder pads. In this case, the amount of solder paste deposition onto the solder pad is crucial to the integrity of the assembled printed circuit board. A wrong amount of solder forms a poor solder joint. Excessive solder paste causes bridging between adjacent pads. Less solder paste causes poor contact. In another example, the wire bonding process connects a wire from the bond pad of a semiconductor die to a lead of a lead frame. This process requires the measurement of the wire loop height and the ball height.
It can be observed that most of the existing on-line methods of measuring the height and volume of an object, such as solder paste, rely on sampling of several points on the deposition. Sampling measurement does not provide sufficient data and information for effective process control. On the other hand, detailed measurement which covers the entire solder paste deposition is time consuming which can only be performed off-line. In the case of measuring the wire loop and ball height in the wire bonding application, manual techniques relying on focusing are commonly used. This technique is difficult to automate and is slow in nature.
Laser structured light and confocal scanning laser microscopy approaches are two common methods for the measurement of object height and volume. The laser structured light approach makes use of a laser line projector to project a finely focused light strip over the solder paste. The displacement of the light strip from its original axis is observed. Using a triangulation principle, the displacement of the light strip is used to calculate the height of an object, for example, the solder paste. U.S. Pat. No. 4,891,772, issued Apr. 15, 1987, discloses point and line range sensors using the principle of laser structured light.
U.S. Pat. No. 4,965,441, issued Feb. 26, 1989, discloses a method of scanning confocal light-optical microscopy. This method measures the focusing of laser beams to determine the height of an object. The entire solder paste deposition can be measured using the scanning confocal light-optical microscopy method. However, this method is time consuming and can only be applied off-line.